Browsers have custom css in their default stylesheet for fieldset elements.

On Chrome it has min-width: -webkit-min-content;

You could just set this rule :

.section fieldset{
    min-width: 0;
}

See fiddle:

http://jsfiddle.net/tg5uk25L/4/

Inspect the elements with Firebug, Chrome Dev Tools, aso to see the difference between the div and the fieldset elements in your stylesheet!